fn main() {
    let s1 = String::from("hello"); // s1 owns the data "hello"
    println!("s1: {}", s1);

    let s2 = s1; // s1 is moved to s2. s1 is now invalid.
    // println!("s1 after move: {}", s1); // This would cause a compile-time error!
    println!("s2 after move: {}", s2);

    let s3 = &s2; // s3 immutably borrows from s2.
    // s2 can still be used, but cannot be mutated while s3 exists.
    println!("s3 (borrow of s2): {}", s3);
    println!("s2 (still valid after borrow): {}", s2);

    let mut s4 = String::from("world");
    borrow_and_print(&s4); // Immutable borrow
    mutate_string(&mut s4); // Mutable borrow
    println!("s4 after mutation: {}", s4);
}

fn borrow_and_print(some_string: &String) {
    println!("Borrowed string: {}", some_string);
}

fn mutate_string(some_string: &mut String) {
    some_string.push_str("!");
}